Does this sound like the ignition switch?

It is possible but I'd suspect bad clamp/ post connections. "Stab" your voltmeter onto the top of the posts and see if it reads voltage. Turn on headlights see if they come on. Re-read post voltage and if it is OK then transfer probes first to one, then the other clamp.

I DID once have a battery die--just like that, way back when

Customer had brought a battery "core" in for exchange on a new battery. I used to sort through the cores a bit and try and charge them and "see what we got." Hell the 15 years I worked in the parts business I only bought 1 battery, the rest were "cores" that turned out to be useable

Anyhow, this little group 22 was nice and clean, and load tested like it could start and aircraft carrier. I dragged it home and a couple days later tossed it into the junk Ford Courier. Ran it ?? about a week I guess and one day it just would not start. Nottin. Must'ha had a break in one of the internal cell connectors. Dragged it back to the core pile, LOLOL